In vehicles such as an EV (Electric Vehicle) and an HEV (Hybrid Electrical Vehicle), a capacitor that supplies electric power to a motor and the like is mounted. In the capacitor mounted in the vehicles, a battery such as a lithium ion battery, a nickel hydride battery is mounted.
In a case where a lithium ion battery is used as the capacitor, when the capacitor is left uncontrolled with being mounted in the vehicle for a long time or when the capacitor is used over a long time, deterioration of the capacitor occurs. The deterioration of the capacitor, as illustrated in FIG. 27, can easily occur when the charge state (SOC: State of Charge, this is a rated value of the remaining capacity of the capacitor with a fully-charged state and a completely-discharged state being set to 100(%) and 0(%)) of the capacitor is high. FIG. 27 is a diagram illustrating an example of the relationship between the duration days (used days) and the capacity decreasing rate of a capacitor. In other words, when the capacitor is used or left uncontrolled in a high SOC state, the decreasing rate of the capacity increases, and accordingly, the deterioration of the capacitor may easily progress.
Similarly, in the state in which the capacitor is in the high SOC state, a usable capacity decreases due to the deterioration of the capacity of the capacitor. The usable capacity is a difference between usable an upper limit SOC (hereinafter, also referred to as a boundary upper limit SOC) and a usable lower limit SOC (hereinafter, also referred to as a usable lower limit SOC). FIG. 28 is a diagram illustrating the relationship between the usage period and the usable capacity of a conventional battery.
As an example of the method for preventing the deterioration of the battery, in order to prevent the battery from being left uncontrolled in the high SOC, a technology of discharging a battery using an externally-connected device or a technology of charging a battery using a timer before the use of the vehicle is known (for example, see Patent Document 1). In addition, a technology of discharging a battery before the lithium ion secondary battery is left uncontrolled for a long time in a charged state at high temperature is known (for example, see Patent Document 2).
In addition, in a case where the deterioration of the capacitor occurs, a technology of decreasing the boundary upper limit SOC of the battery in accordance with the state of deterioration of the battery, prohibiting idling or limiting accessory driving is known (for example, see Patent Document 3). In addition, a technology of controlling the SOC so as to maximize the vehicle output in accordance with the state of deterioration of the battery is known (for example, see Patent Document 4).